148 



B. A. AFZELIUS 



Fig. I. The two RNA-containing bodies of microscopic dimensions in the egg cytoplasm can be compared in this 



picture. A portion of a "yolk nucleus" with a-cytomembranes enclosing a few mitochondria is seen at the right and two 



triangular heavy bodies to the left of this inclusion. The picture is from the sea urchin species Styungy/ocentroriis 

 droehacliiensis. Magnification ■ 30,000. 



chondria were not or very faintly stained by this 

 technique. 



Another cell component that stained with the 

 RNA-specific stains was found occasionally in some 

 oocytes of Swedish sea urchins and regularly in the 

 oocytes of the species Strongylocentrotus droebachien- 

 sis. This particle can be described as an elliptical 

 body consisting of concentric membranes which 

 often enclose a particle resembling a yolk granule 

 or a few mitochondria. The total diameter is 5 /< 

 or less. When centrifuged, these bodies go to the 

 clear layer. They are sometimes called "yolk nuclei", 

 which is not a particularly happy name as this term 

 has been used for different structures in different 

 oocyte types and as they have probably nothing to 

 do with yolk synthesis. 



Electron microscopical observations. — Two of the 

 RNA-containing structures described above were 

 recognized with certainty in the electron micro- 

 scope on ultra-thin sections; the "yolk nuclei" of 

 the oocytes and the heavy bodies of the mature egg. 



In the "yolk nucleus", whose membranous struc- 

 ture is vaguely discernible with the light microscope, 

 the individual membranes were seen to have a 



thickness of 60 A and to be associated with granules 

 approximately 150 A in diameter. The membranes, 

 like those of the exocrine pancreatic cells (20) were 

 apparently arranged in pairs, with the granules 

 located on their outer surfaces. The yolk granules 

 and mitochondria that were typically enclosed by 

 these membranes exhibited no peculiarities in their 

 fine structure. 



Turning to the mature egg the heavy bodies were 

 examined in sections from the centrifugal pole of a 

 stratified egg. Each heavy body was seen to be a 

 dense aggregation of tiny granules (approximately 

 150 A) surrounded by a membrane of exactly the 

 same appearance as the nuclear membrane. The 

 shape of the heavy bodies as seen in sections was 

 variable; the type most often observed possessed a 

 regularly triangular outline, but bodies of a lentiform 

 outline were also common, and some, especially the 

 larger ones, appeared polygonal or rounded. The 

 membranes did not seem to enclose the granular 

 mass completely, but left small gaps at the angles. 

 Occasionally particles of the same appearance have 

 been found on the nucleo-cytoplasmic border in 

 mature eggs or in oocytes. They were enclosed by a 



